1. Field of the Invention
The present invention relates to a dry etching apparatus and method, and more particularly to a dry etching apparatus and method which are suitable to implement high-selectivity high-anisotropy etching at high speed and with high throughput.
2. Description of the Related Art
Conventional dry etching techniques have used low gas pressure of 1-10 mTorr or so in order to improve anisotropy. Discharging gas with such a low gas pressure requires adoption of effective discharging techniques. One of them is microwave discharging. Examples of microwave gas discharging are disclosed in "Journal of Electrochemical Society" 1982, page 2764, "Journal of Vacuum Science Technology" A7, 1989, page 899 and "Proceeding of Dry Process Symposium" 1990, page 99.
Generally, reducing pressure lowers etching speed, so the conventional dry etching at low gas pressure uses high density plasma with the degree of ionization being high. The higher the plasma density, the higher the etching speed because the ion current incident to a sample to be treated increases. In microwave discharging, the plasma density can be made high by boosting the power of the microwave.
As an alternative way of improving the anisotropy, the conventional dry etching adopts an exchange of gas. For example, the process disclosed in JP-A-61-61423 and JP-A-63-65628 uses different gases in different steps in such a manner that the first step performs an anisotropic etching, the second step forms a side wall protection film and the third step performs an isotropic etching. The process disclosed in JP-A-60-50923 and JP-A-2-105413 realizes anisotropic treatment by making "time modulation etching" of exchanging etching gas for deposition gas at intervals of a few seconds. The process disclosed in JP-A-2-270320, in order to improve the temperature controllability in lower temperature etching to thereby increase the anisotropy, fixes a wafer by electrostatic adsorption, requires a plasma which (as does the wafer removal). Discharging gas exchanged into inert gas assures more accurate etching. Thus, the efficient method of improving the anisotropy has been to exchange gas.
A dry etching apparatus with the short gas residence time of 25 ms is disclosed in "Journal of Vacuum Science Technology" B8 (1990) p. 1185. The apparatus has a volume of about 2 liters between electrodes and an effective exhaust speed of 80 liter/sec.
The prior art described above has the following problems to be solved. The conventional dry etching techniques show a phenomenon that even when the incident ion current density is enhanced with the density plasma made high, the etching speed of a sample ceases to increase. This causes a problem that the necessary etching speed cannot be obtained solely by making the plasma density high. The etching speed can also be increased by applying an RF bias to a sample or body to be treated to thereby enhance the energy of incident ions. Enhancing the incident ion energy, however, deteriorates the etching selectivity ratio of the sample and a mask or underlying layer.
For example, the Si gate treatment process in which a resist is used as a mask, a poly-Si (polycrystalline silicon) sample having an underlying SiO.sub.2 is to be treated and, requires a poly-Si/resist selectivity ratio of 5 or more and a poly-Si/SiO.sub.2 selectivity ratio of 50 or more. This treatment process uses Cl.sub.2 gas plasma for etching. In order to provide the above selectivity ratios, the plasma etching was performed at the poly-Si etching speed of 300 nm/min or so. Since the poly-Si film is about 300 nm thick, the treatment time including 50% over-etching was 1.5 minutes. But the treatment time is desired to be one minute or less for good throughput. Therefore, the first problem to be solved is:
(1) to realize a poly-Si etching speed of 450 nm/min or more with a poly-Si/resist selectivity ratio of 5 or more and a poly-Si/SiO.sub.2 selectivity ratio of 50 or more. PA0 (2) to realize a poly-Si etching speed of 450 nm/min or more with a poly-Si/resist selectivity ratio of 10 or more and a poly-Si/SiO.sub.2 selectivity ratio of 100 or more. PA0 (3) to make the etching without leaving, a material with a low gas pressure as residue. PA0 (4) to estimate and control etching uniformity before the etching. PA0 (5) to introduce gas in a pulse shape with a pulse width of 0.1 ms to 100 ms into a treatment chamber. PA0 (6) to improve throughput in the etching accompanied by gas exchange. PA0 (1) Means for solving the first problem PA0 (2) Means of solving the second problem PA0 (1). If the ion energy is reduced to make the selectivity ratio twice, the etching speed of poly-Si becomes 200 nm/min. Thus, in order to make the etching speed 2.3 times as high as 200 nm/min to provide 450 nm/min, the ion current density must be raised to 27 mA/cm.sup.2 which is 2.3 times as high as that in the item (1). This can be realized with a power surface density of applied high frequency of 10 W/cm.sup.2 or more to provide an high density plasma. PA0 (3) Means of solving the third problem PA0 (4) Means of solving the fourth problem PA0 (5) Means of solving the fifth problem PA0 (6) Means of solving the sixth problem
With development of miniaturization of semiconductor devices, the resist mask and the underlying SiO.sub.2 is expected to become about half as thick as at the present time. On the other hand, since the thickness of the poly-Si gate is expected to remain fixed, selectivity ratios which are twice as large as before are required. Therefore, the second problem to be solved is:
The conventional dry etching technique has a disadvantage that if the sample to be treated contains plural kinds of atoms like AlCuSi, a material with a low gas pressure such as a reaction product of Cu is likely to be left as residue. Therefore, the third problem to be solved is:
Uniformity in etching depends on the uniformity in the density of the ion current supplied to the sample. But the conventional dry etching has no means of examining the uniformity in the ion current density. Thus, the uniformity in etching cannot be known until the etching is completed. The fourth problem to be solved is:
The conventional dry etching apparatus, in which the gas residence time within a chamber was 0.4-3 sec or so, could not introduce gas in a pulse shape into the chamber at time intervals shorter than the residence time which is the time that the gas resides within the chamber from when it is supplied into the chamber to when it is exhausted. The gas residence time can be calculated by EQU gas residence time=(volume within the apparatus)/(effective exhaust speed) (1)
In order to implement "atomic layer etching" in which a sample is etched at a sufficient etching speed (100 nm/min or more) for each atomic layer, at a pressure of 0.1 mTorr, it is necessary to control gas adsorption with the accuracy of at least 10 atomic layers, preferably 0.01 atomic layers, for the surface of the sample. In order to control the gas adsorption with the accuracy of 10 atomic layers, it is necessary to introduce the gas with a pulse width of 100 ms, and in order to control the gas adsorption with the accuracy of 0.01 atomic layers, it is necessary to the gas with a pulse width of 0.1 ms. The fifth problem to be solve is:
The conventional dry etching apparatus, in which the gas residence time in the treatment chamber is 0.4-3 sec or so, took one second or more to exchange gas. The etching accompanied by gas exchange has a problem that it takes relatively longer time to exchange the gas to thereby reduce the throughput. In order to observe or examine the shape of the sample during the etching by an observing means attached to the etching apparatus, the gas supply must be stopped to reduce the gas pressure. It took a few seconds to exhaust gas so that the etching with observation of the sample shape reduces the throughput. Therefore, the sixth problem to be solved is: